I know not a lot of people accept this... But the easiest way is to change engine... I'm driving a W124 with a Toyota 1JZ engine... My car weight more than 1800kg and I can easily smoke any joker that tries to tailgate me... Not telling you that this is the way to go but for me this is the way... Much more power plus I retain the smoothness of the inline six engine... Win win situation for me but maybe not those who wants pedigree... If you're trying to mod the original engine, my suggestion is to just forget about it...